The present invention relates to a method of determining guidance errors and compensating for their influence on the precision of positional determination of a guided component of a machine, such as a multiple-coordinate measuring machine.
In principle, various guidance errors can occur for a guided part, in relation to a single guide therefor, namely three angular displacements of the guided part, about an axis parallel to the guide and about each of the two axes perpendicular thereto, as well as linear offset of the guided part in the respective directions of the two last-mentioned orthogonal axes. Furthermore, the scale used for positional determination of the guided part in the direction of guidance can have errors in graduation.
These guidance errors can be reached to two classes: The first class comprises so-called long-period guidance errors which are due primarily to deviations in planarity of involved guidance surfaces and to deformations of the guidance elements (bearing +guide) as a result of changes in load. The second class comprises short-period guidance errors which, as a rule, do not exhibit reproducible behavior. The two types of errors affect the precision of positional determination to different extents, depending on the construction of the guide. For example, the short-period error is negligible for air-mounted guidance systems; but in systems which rely on anti-friction bearings, the bearings can make a substantial contribution to the total error.
To compensate for the influence of guidance errors one can, in principle, proceed along two different courses: either one compensates for the guidance errors themselves, or one detects them and corrects the result of the measurement for the applicable position position of the guided part.
The first possibility is utilized in West German Auslegeschrift AS 2,231,644, West German Auslegeschrift AS 1,157,877, West German Pat. No. 1,915,940 and West German Offenlegungsschrift OS 2,647,147. These solutions however, require a large mechanical expenditure since precise correction movements are imposed upon guide elements which are subjected to rather considerable loads.
West German Pat. No. 1,638,032 discloses an automatic program control in which the measured coordinate values of the guided part are provided with a position-dependent correction. Error values used for the correction are determined by the actual measurement process and are stored in tabulated form in a computer. Then, the stored values are used to modify the control commands of the program by which the guided part is automatically positioned.
Such program control requires relatively great electronic expense, since a multi-dimensional data field for the correction must be stored, to serve positioning in each of several coordinates. Since control commands for the machine are derived from the stored correction values, speed of machine operation is dependent on the speed of computation.
The second method (correction of the measurement result) is used in devices described in West German Pat. No. 1,964,470 and in West German Auslegeschrift AS 2,248,194.
In West German Pat. No. 1,964,470, all displacement axes of a machine are provided with afocal optical systems to detect the guidance errors. Each of these optical systems images two pairs of marks or scales on the other. But each afocal optical imaging system requires a relatively large aperture in order to obtain sufficiently high resolution. Thus, to employ such optical systems is not only expensive and uneconomical, but also considerably increases the dimensions of the machine in question, and such increases are generally undesired.
West German Auslegeschrift AS 2,248,194 describes a portal-type measurement machine which is provided with two scales measuring in the direction of portal displacement, one scale for each of the two columns of the portal.In this way, only angular displacement of the portal about the vertical is detected, and there is no detection of lateral offset, i.e., offset perpendicular to guidance direction. Furthermore, machine cost may be increased by the additional scale.